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Aaronut♦Oct 29 '10 at 14:17

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The tears come only after you BLINK. If you get used to keeping your eyes open while cutting onions, it helps a great deal. Other trick is not to expose the the cut side of the onion by putting on the cutting board and holding the onion together.
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MandoMandoDec 3 '10 at 18:46

19 Answers
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The tearing of the eyes is a result of enzymes that form a sulpherous gas when the onion is cut (concentrated inside the inner core or bulb of the onion) being released into the surrounding air.

You have a few options to avoid this - one would be to not cut through the center of the onion but to extract the "core". This is kind of a pain and means you are discarding a decent chunk of onion, so I don't like it too much.

Another option is to use a fan to blow the gases away from you. A small fan next to you blowing horizontally should do the trick.

You could also wear goggles, although you'd have to leave them on for a while until the gas dissipates from the area.

Another technique is to cut a lemon in half and rub the fresh lemon against both sides of the blade. You'll have to keep "refreshing" the juice coat and your onions will have some lemon juice on them, so this is somewhat limited by that factor.

I knew that those safety goggles they made me buy for grade 9 science class would come in handy someday!
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Aaronut♦Jul 11 '10 at 3:19

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just to be clear..this is caused by a gas and we tear because we inhale this or this gas goes into our eyes?
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CodeToGloryJul 17 '10 at 16:30

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@CodeToGlory: It goes into our eyes. When it combines with the moisture in our eyes it forms a weak solution of sulfuric acid. This stuff burns. :)
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hobodaveJul 17 '10 at 20:19

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There isn't a 'gas in the onion that is released'. There is a compound in the onion which undergoes chemical reactions when cells are burst. This compound is volatile, and irritates the eyes. There is no sulfuric acid. See loc.gov/rr/scitech/mysteries/onion.html
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Adam ShiemkeJul 19 '10 at 16:49

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@Adam Shiemke: Yes, it does undergo a reaction when the cells burst, and true, there's no sulfuric acid, But 'volatile' means that it vaporizes (becomes a gas) easily, so it is indeed released in gas form. It does also contain sulfur. So saying 'a sulfurous gas is released' is pretty much completely accurate.
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Jefromi♦Aug 16 '10 at 13:46

Can't comment directly due to rep limit, but I'm not sure Ryan Elkins above is correct. I'm fairly certain both allinase and LFS are found throughout the layers, though the "core" might contain a higher concentration. Certainly there's no gas contained in the central bulb.

There's a couple of basic strategies for avoiding eye irritation:

Avoid breaking cell walls: use a sharp knife

The enzymes and amino acids involved in producing the Onion Lachrymatory Factor (seriously!) are normally contained in the cells, and only become a problem when released in volume. Using a sharper knife will avoid mangling cell walls, creating more clean separations between layers, and reducing the amount of enzymes released.

Slow down the reaction: refrigerate/freeze the onion

I would guess the activity of both enzymes involved here peaks somewhere around room temperature, or maybe a little higher. You want to get out of this optimum range: either freeze the onion, or heat it beyond denaturation. The former is probably way more practical, since heating to denaturation will leave a soggy mess instead of an onion. You may also be able to deactivate the allinase by substantially altering the pH, for example by coating your knife in lye (not recommended) or lemon juice.

Avoid the reaction products: cut underwater or ventilate

The OLF and the reaction intermediaries look to be fairly water soluble, so cutting under running water should take care of them with sufficient flow rate.

I'm not sure what the mechanism behind the common candle suggestion is, OLF is a thial oxide and not very flammable. My best guess is that the rising air draws some of the irritant up and away from the area, but in that case a fan should work much better.

A sharp knife does wonders -- you don't burst as many cells, and you can get done with the chopping faster. I hate chopping onions at my friend's places, as they don't keep their knives sharp enough.
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JoeJul 11 '10 at 4:32

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The sharp knife part is key. In fact - I know when my favorite knife is getting a little dull when I catch more of the onion gas then I used to.
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NickJul 16 '10 at 19:34

Definitely a sharp knife just make sure its a normal blade not serrated as that can make things really bad.
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Mark DavidsonJul 16 '10 at 19:46

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The candle probably doesn't 'burn' any of the gas off, but creates a small air current that draws air in and quickly up away from your face. Even an open window with an imperceptible breeze can make a huge difference.
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Nick TOct 22 '10 at 18:28

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That explains why a food processor is much worse for this than cutting them with a knife: it's blunter.
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Marcus DowningDec 19 '10 at 15:36

Stick it in the freezer for half an hour first. I just discovered this by accident (I went to get bacon out of the freezer and absent-mindedly put the onion I was holding in the freezer at the same time!) Alternatively, you can put it in the refrigerator, but that's slower, and if you leave it there too long the flavor mellows. Either way, you're making the problematic compound less volatile, so less will end up in the air to irritate your eyes.

The crying chemical is a gas; you're likely dispersing the gas by walking to the sink to rinse them off ;)
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mattbastaJan 2 '11 at 18:12

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@mattbasta, the sink is right beside me. I think the water actually washes away the acid that forms the gas, before it can turn into gas.. Wikipedia backs me up on the water idea, anyway.
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BlorgbeardJan 3 '11 at 0:04

At a recent ICE Knife Skills 1 class in NYC this is what the instructor (Brendan McDermott) recommended. Apparently the lateral cuts in general are what lead to the highest release of the gases.
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alexpotatoFeb 27 '14 at 14:27

This is the standard technique for chopping an onion. I suppose it Works for dicing as well, simply putting more distance between each cut. It's a good way to cut it, but not enough to stop the crying game, for me anyway...
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The DagJan 10 at 14:49

America's test kitchen did a segment on this where they tested out a number of different methods including the various folk remedies that people claim works. The only method that they found to reliably work was wearing goggles (you can even buy special goggles specifically made for cutting onions). I don't know if they tested the open flame method or not.

Any method that does not physically prevent the gasses released from the onion from entering your eye simply will not work. The crying is the result of irritation from a chemical reaction when the gasses released by a cut onion interact with your tears (I believe the reaction creates small amounts of sulfuric acid, but don't quote me on that). So you either need a physical barrier around your eyes that keeps the gasses out, or some sort of chemical barrier that will react with the gasses before they reach your eye and therefore prevent them from reacting with your tears.

Of the various answers presented here, the only ones that would seem to have any chance of actually being successful are goggles, a fan (that moves the gasses away from you before they get to your eyes), or possibly an open flame. Of those three, the goggles are the only sure-fire way, as the movement of air is difficult to predict and control.

Here is an article that lists quite a few interesting ways to avoid it. One I think is worth mentioning is to cut the onions under water. I've also read that putting the onions in water for 30 minutes before cutting also helps. Both of these methods would help reduce the compounds that are released when chopping and stop them from going after your eyes :)

Does putting them in water affect the flavour at all?
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lomaxxJul 11 '10 at 0:25

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IMO after soaking for 30 mins. they still taste plenty like onions to me :) Your taste might be different so you might want to try it out first. I imagine if you cut them under water as opposed to soaking any impact on the taste would be greatly reduced!
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KrypticJul 11 '10 at 0:57

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The only way to lessen the taste of an onion is to put it in a bowl with water and ice. That way the taste will fade a little bit - but that's another question.
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GeorgeJul 17 '10 at 19:55

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Washing cut onion improves the flavor a bit. The same stuff that stings eyes tastes bad, so rinsing it off improves taste. If you prep onions ahead of time, you should rinse them for this reason.
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Adam ShiemkeJul 20 '10 at 1:07

Simply breathe only through your nose. Do not talk or open your mouth while chopping. I never cry when chopping onions anymore and haven't in years. I think the bread thing is just a way to keep your mouth closed. (Same goes for holding water or a spoon or a match in your mouth, or chewing gum.)

Yes, this! I agree that the "bread in your mouth" method is really about keeping your mouth shut, as is the "water in your mouth" method. Since I figured this out (not having bread to put in my mouth one day so I just kept it shut) I haven't shed a single onion tear.
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Matthew FrederickDec 14 '10 at 8:54

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I go for the beer in mouth method. Of course you tend to swallow and then need more. Works for me.
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WW.Aug 19 '13 at 11:42

Learn to cut onions faster. Seriously. Unless you're working in a restaurant kitchen and will have to chop more than a dozen (or hundred) of onions, you should be able to chop it fast enough and store them or cook them right away. Then take a few steps back until the gas dissipates from the cutting area.

If I have several to do, I peel them all and cut them in half. Then I do all the chopping at once. You don't need to be a knife master. Just good enough and have a good knife.

I know most folks here are probably pretty comfortable with a knife but it may be irresponsible to recommend cutting faster. Sloppy chopping can lead to a lot of accidents. I imagine most people already cut at a pace they feel comfortable.
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eyelidlessnessNov 7 '10 at 20:21

For me, wearing contact lenses works perfectly. I can cut onions as much as I like in them, and never cry. To be detailed, they are AirOptix Night&Day, if someone wonders that some contact lens types maybe don't work.

Keep your face away from the onion. Seriously! If you just move your head so it isn't directly above the onion when you cut it, the gasses that would normally go into your eyes won't and you won't cry. One way to do this is simply to sit down.

When I worked at subway we had to cut all kinds of onions. This is what worked for us:

Wet a paper towel, fold it (hot dog style), and place it under your eyes and over your nose. You have to make sure you don't lean down too much or the towel obviously falls off. Also, it helps to have a large nose.

Then, if we either neglected to do that, or we simply had to cut too many of them, stepping inside the walk in freezer for just 30 seconds kind of re-sets your tears and buys you another 5 minutes or so of tear free cutting.

Obviously most people don't have a walk in freezer, but i imagine sticking your face in a normal one will work equally as well.

My mother suffers terribly when chopping onions. Her solution is to chop them outdoors. It happens that there's a waist-height coal scuttle a couple of paces from the kitchen door, so she does it on that.